I think this will work.
1) Windows Menu --> Preflight Verifier
2) Preflight Verifier window --> top right corner choose pdf 1.5 (this supports transparency) from postscript.
3) File Menu --> Export --> PDF
I replicated your issue and this seemed to fix it, let me know.
There are quite a few pdf options, depending on your needs you might want to read up on them or ask I will try to assist you.
Hope this helps,
mnawij